New Zealand: 40 killed in 'terrorist' attack on two mosques

New Zealand Prime Minister Jacinda Ardern says 40 people were killed and more than 20 seriously wounded in mass shootings at two mosques in central Christchurch on Friday.

New Zealand has been placed on its highest security threat level, she said, adding four people with extremist views are held in police custody but they had not been on any police watchlists.

One of the suspects is Australian, Australian Prime Minister Scott Morrison said. "I can confirm that the individual who was taken into custody I have been advised is an Australian-born citizen," he told reporters in Sydney.

Video footage widely circulated on social media, apparently taken by a gunman and posted online live as the attack unfolded, showed him driving to one mosque, entering it and shooting randomly at people inside.

One man who said he was at the Al Noor mosque told media the gunman was white, blond and wearing a helmet and a bulletproof vest. The man burst into the mosque as worshipers were kneeling for prayers.

"He had a big gun ... he came and started shooting everyone in the mosque, everywhere," Ahmad al-Mahmoud told Reuters. He said he and others escaped by breaking through a glass door.

"This is one of New Zealand's darkest days," Prime Minister Ardern said.

Police advised people living in the city center to stay indoors. All mosques, schools and council buildings were also placed on lockdown.

The Bangladesh cricket team was also at the mosque attending Friday prayers. The team was slated to play New Zealand in a third cricket test starting on Saturday.